Screening in papermaking means separation of foreign or different particles from a pulp suspension and may be done in different process steps in a fiber line.
Pulp from cooking inevitably contains unwanted solid material. Some of the chips may not have cooked properly, and some of the fibrous material may not be completely in the form of individual fibers. Defects in wood may show up as non-fibrous solids. Contaminants other than wood may also enter cooking with chips. The purpose of screening is to remove these impurities from the main pulp stream for reject treatment as efficiently as possible and wash off the good fibers, return these to the main pulp stream and prepare the reject for treatment.
A screening apparatus, see e.g. EP0 444 051, may comprise a screen housing enclosing a cylindrical barrier, called a screen member, having apertures in the form of e.g. holes or slits for the actual screening. The screening apparatus may further comprise an inlet for the pulp suspension leading into the screen member, an accept outlet for accept, i.e. material with small particles that has passed through the screen member from the inside and out, and a reject outlet for reject, i.e. material with large particles which has not passed the screen member.
To improve the screening, the screening apparatus may e.g. be provided with some sort of rotor having pulsations members, e.g. blades or protrusions. Said rotor rotates inside the screen member or else the screen member may be rotating. Further, the screen member may be provided with different variants of protrusions on its inner surface.
In the prior art section of EP 205 623 there is shown an open solution with wings attached to a “spider” and a reinforcement ring. The loose structure is very unstable and not sufficiently rigid to withstand the forces especially since the wings are attached with screws or bolts. A similar loose, unstable structure is shown in U.S. Pat. No. 4,328,096.
In EP 444 051 and EP 868 564 the rotor is designed as a cylindrical rotor body having arms with pulsation generating members, also called blades or wings, which are attached on the mantle area of the rotor body. They have complicated manufacture, since a complicated fixture is needed in order to position the wings radially and circumferentially. Several complicated fixtures are needed to be able to handle different screening apparatus sizes as well.
It is also complicated to change the wings, when they are worn or if you want to change the wing design e.g. in order to optimize the energy consumption. In order to get sufficient precision, the wings are preferably cast, which may cause strength problems. Further, cast wings may be warped and have to be aligned. Cracks may occur in cast wings.